Generally this invention relates to a two-part adhesive composition for use in bonding elastomer articles, the bonded articles per se and the method of using the adhesive therewith. The adhesive is also useful in bonding plastics and metals.
More particularly it relates to use of the adhesive composition in the manufacture of retread tires, particularly truck tires, and the resultant retread tires as articles per se. Additional applications include bonded conveyor belts, molded rubber parts, patch kits for repair of molded rubber articles and the like.
The adhesive is a polyurea based two-component system curable at room temperature or slightly elevated temperatures with outstanding adhesion to molded elastomer articles. It is particularly successful in the manufacture of retread SBR (styrene-butatiene rubber) tires.
Heretofore, retreading has been typically accomplished commercially by use of a urethane-based bonding system which uses uncrosslinked rubber or "cushion gum" cured in an autoclave process. The autoclave process is cumbersome and requires a heat cure of 3 hours at 250.degree. F.
U.S. Pat. No. 3,755,261 to Van Gulick and U.S. Pat. No. 3,876,604 to Caruso disclose a prior art polyurethane/urea system that uses metal halide salts of aromatic diamines to cross-link isocyanate terminated polymers. Specifically disclosed is the use of 4,4'-methylene dianiline/NaCl salt in dioctyl phthalate mixed with an isocyanate terminated prepolymer. When mixed they form a relatively stable mixture at room temperature that can be used as an adhesive. The mixture is activated when heated to 100.degree. C. or above. The salt dissociates and the MDA is then available for reaction with the isocyanate moiety. This forms the final crosslinked polyurethane/urea.
Because the system has to be heated to crosslink, several methods were later developed to initiate the reaction at ambient temperatures or slightly elevated temperatures by lowering the temperature at which the salt dissociates. These are disclosed in the following patents: U.S. Pat. No. 3,888,831 to Kogon describes the use of additive amounts of a polarizable compound such as benzene or a polar compound without reactive hydrogens as accelerators of the above mentioned reaction. The additive acts to dissociate the salt at a lower temperature.
U.S. Pat. No. 3,891,606 Kogon describes the use of active hydrogen donor additives such as alcohols, phenols, polyols, primary and secondary amines, and specifically 1,4-butanediol and glycerol.
U.S. Pat. No. 4,046,743 to Schonfield describes the use of high dielectric compound additives, exclusive of halogens, such as oleic acid, benzaldehyde, adiponitrile, acetic anhydride, polymethylsiloxane and diethyl sulfate having a dielectric constant of 2 or greater. Cure temperature range with these compounds is 30.degree.-140.degree. C.
U.S. Pat. No. 4,435,456 to Hausch describes a process and composition that uses MEK as a dissociation material for the salt.
U.S. Pat. No. 4,517,331 to Parket et al claims the use of pseudo crown ethers having the formula --(y--CH2--CH2)n where y=N,S,O and n is 3 or more.
U.S. Pat. No. 4,772,676 to Koch discloses the use of alcohol catalysts for the acceleration of the salt, -isocyanate prepolymer reaction as an improvement over using ketones.
The following patents teach the application of the above mentioned technology for adhesives:
U.S. Pat. No. 4,125,522 to Becker discloses the use of an MDA salt to crosslink an isocyanate terminated prepolymer as an adhesive used for bonding shoes with application use temperatures to 70.degree. C.
U.S. Pat. No. 4,247,676 to Kimball discloses the use of an MDA salt for crosslinking an isocyanate terminated prepolymer which is catalyzed with a dicarboxylic acid of 4 to 10 carbons for application as an adhesive to bond SMC.
U.S. Pat. No. 4,435,456 to Hausch discloses the use of an MDA salt for crosslinking an NCO terminated prepolymer that uses MEK as a room temperature accelerator for bonding tire treads and making tire patches in the repair of tires. The elastomer is treated with a solution of trichloroisocyanuric acid.
Likewise the following patents address the use of metal halide salts with NCO terminated prepolymers for bonding, patching or otherwise repairing elastomers:
U.S. Pat. No. 4,434,832 to Koch; U.S. Pat. No. 4,485,135 to Koch; U.S. Pat. No. 4,485,136 to Koch et al; U.S. Pat. No. 4,544,427 to Hausch; U.S. Pat. No. 4,696,332 to Koch; U.S. Pat. No. 4,765,852 to Koch; U.S. Pat. No. 4,772,676 to Koch.
The prior art described herein is not satisfactory because of the need to agitate the salt containing material prior to use, the lack of means to eliminate the DOP from the system and the inconsistent cure based on the need to add materials that cause the salt to dissociate at a lower temperature. The use of polyurethane prepolymers also has a negative effect on the adhesion of the system at elevated temperatures as would be encountered in dynamic applications such as tire retreading, tire patching and repair, and conveyor belts.
In the case of the present invention, outstanding adhesion is obtained by using in place of the prior art polyurethane prepolymer a polyurea or mixed polyurethane/urea prepolymer.
As is already known in the art, a surface treatment is required to effectively bond adhesive to any elastomer. The treatment may be accomplished by sulfuric acid etching or by the application of a primer such as those described in U.S. Pat. Nos. 4,485,135 and 4,485,136. A particularly preferred primer for use on SBR (styrene-butadiene rubber) and the like is 2% 1,3-dichloro-5,5-dimethyl hydantoin in methyl chloride. Others are known in the art and may be used in conjunction with this invention.